Suspension apparatus is provided in existing vehicles to at least partly isolate a suspended mass of the vehicle from irregularities in the surface on which the vehicle is to travel. Such isolation provides a smoother ride than would otherwise be the case for passengers that form part of the suspended mass. Existing suspension apparatus also aims to resist undesirable changes in attitude of the suspended part of the vehicle that tend to occur during acceleration, braking and during changes in the direction of travel. Such changes in attitude, sometimes referred to as “body pitch” and “body roll”, are undesirable in reducing the comfort of passengers and also in reducing the performance of the vehicle, which, at exaggerated attitudes, may become unresponsive or unsafe.
Suspension apparatus can generally be separated into two categories: passive suspension and active suspension. In passive suspension apparatus, characteristics of the apparatus cannot be varied during operation. By contrast, in active suspension apparatus, some characteristics can be varied, usually in an attempt to avoid more completely the undesirable phenomena identified above. For example, in some active suspension apparatus, additional energy can be supplied to the apparatus to adjust the ride height of the vehicle independently and dynamically at each point of suspension and thereby reduce body roll. In other active suspension apparatus, which are perhaps more properly termed “semi-active suspension”, the viscosity of fluid in dampers of the apparatus can be varied dynamically to change the rate at which the suspension rebounds. The rebound rate can be varied in this way during operation to provide ride characteristics that adjust to account for changes in the operating conditions of the vehicle, for example, changes in the surface on which the vehicle is travelling, the speed of the vehicle or whether or not the vehicle is cornering. Though active and semi-active suspension has met with some success in improving the performance of suspension apparatus, such arrangements are generally complex and expensive, and can also be large and heavy. For example, the use of electro-hydraulic components to provide the suspension in active suspension apparatus results in high initial cost, significant energy consumption during use, high servicing cost and difficulty in packaging those components within the vehicle because of higher weight and larger size than their passive equivalents. Semi-active arrangements that use magnetorheological dampers have a lower energy consumption during use, but do not generally perform as well as fully active arrangements.
There is therefore a need to provide suspension apparatus that exhibits at least some of the advantages of active or semi-active suspension, but that avoids at least some of their drawbacks.